Power amplifiers are utilized in radio-transmitter circuits for generating a signal with relatively high power to be fed into an antenna. In order e.g. to avoid excessive energy consumption and/or to avoid unnecessary dissipation of heat, a relatively high efficiency is normally desired.
One way of obtaining relatively high efficiency is to use one-bit modulation in combination with a switched-mode power amplifier. However, the use of one-bit modulation introduces a relatively large amount of quantization noise. To counteract the quantization noise, the switched-mode power amplifier may be connected to a load via a band-pass reconstruction filter, the purpose of which is to filter out at least part of the quantization noise power. The band-pass reconstruction filter normally presents a load to the amplifier that varies with the duty-cycle. Since power amplifiers are normally matched for high efficiency at peak output power, this typically results in that the efficiency will drop for low duty cycles.